As citizens of Boston, and the rest of the U.S., slowly come to terms with Monday’s horrific attack, tributes have continued to pour in from around the globe. World leaders joined celebrities and ordinary well-wishers to express their deep sorrow for victims of the twin-blasts that claimed the lives of three and injured at least 176 people during the city’s annual marathon celebration.

Canadian Prime Minister Stephen Harper called the attacks “deeply troubling” as he arrived in London for the funeral of former British Prime Minister Margaret Thatcher. Security has been ramped up in the U.K. with some 4,000 police officers brought in to marshal protesters and VIP dignitaries for the funeral on Wednesday, reports Sky News.

London Mayor Boris Johnson called the Boston attacks “shocking, cowardly and horrific” as his city prepares to host its own equivalent race this Sunday. A 30-second silence will be observed before each of three starts of the London Marathon and the 37,500 competing runners will be encouraged to wear black ribbon in solidarity with those across the Atlantic.

“We are determined to deliver an amazing event that will focus on one of the core pillars of the London Marathon, which is to have fun and provide some happiness and a sense of achievement in a troubled world,” race director Hugh Brasher told the Guardian. Meanwhile, organizers of the Rio 2016 Olympics reiterated that security is a “top priority” and offered their “deep thoughts and condolences to everyone affected by this tragic incident.”

Scores of people were severely maimed during the Boston bombing with preliminary police reports indicating that the two bombs were created out of home pressure cookers packed with ball bearings and nails. Doctors in the city carried out numerous amputations of legs and feet in the aftermath of the tragedy.

Indian Prime Minister Manmohan Singh emphasized his country’s unity with the U.S. when condemning the attack. “We stand in solidarity and sympathy with the bereaved families, the injured and the people of the United States,” he said via Twitter. Meanwhile, Indian media noted that no Indian nationals were hurt in the deadly explosions, the worst terrorist attack on U.S. soil since September 11, 2001.

Chinese President Xi Jinping expressed his “deep sympathy and sincere condolences” to President Barack Obama and the American people, reported China’s state-run Global Times. One of the three fatalities on Monday is understood to be a Chinese graduate student, but she has apparently not yet been named upon the wishes of family members.

Users of China’s Twitter-like social media service Weibo reportedly praised the U.S. response to the incident. “In the face of the tragedy, we can learn a lot from the American government, media, businesses and citizens’ kind interactions,” wrote one user, according to the International Business Times.

Australian Prime Minister Julia Gillard expressed her sympathy for victims of this “senseless and very, very, cruel attack” that targeted families enjoying the Patriot’s Day public holiday. “Australia unreservedly condemns this brutal and senseless attack on what is a great event known around the world,” she said. The Australian newspaper described the efforts of countrymen attempting to return home in the wake of the bombing.

Afghan President Hamid Karzai said both he and the people of Afghanistan empathized with the citizens of Massachusetts. “Having suffered from terrorist attacks and civilian casualties for years, our people feel better the pain and suffering arising from such incidents,” he said in a statement. Adel bin Ahmed Al-Jubeir, Saudi Ambassador to the U.S., described “a heinous crime which contradicts the values of humanity.”

Former Mexican President Felipe Calderón was watching the marathon just a few miles from where the two explosions ripped through the heart of the city. “My solidarity is with the victims of the explosions of today in Boston and with the American people that unjustly suffers these acts,” he wrote in a Twitter post. Calderón is spending the year as a fellow at nearby Harvard University.

A total of 158 French nationals had signed up for the Boston Marathon and French President François Hollande asserted “France’s total solidarity” with the American people in seeking justice for the atrocity. Meanwhile, the German Embassy in the U.S. on Tuesday was still trying to track down the nearly 250 Germans registered as runners, according to The Local, as German Chancellor Angela Merkel described being “horrified” by what transpired. Germany had one of the largest foreign contingents participating in the race.